The GBG (Greatest British Glaciation) -- another clue

An Teallach, the highest ridge of mountains in Wester Ross -- 

with traces of at least two glaciations on the slopes and plateau remnants, and summits 

with no trace at all of overriding ice.

This is an old article, c 23 years old, but nonetheless valuable.  it uses a range of geomorphological and glaciological methods to collect data and reconstruct glacial events.  It seems to me to be an impressive and reliable piece of work.  The authors have measured trimlines on many of the high summits of Wester Ross, with till and other glacial traces below them and largely periglacial blockfields, bare rock and scree above.   The ice surface characteristics interpreted from a mass of data make perfect sense, and accord with other evidence from outside this area of study.  On the map below we can see high ground, troughs and valleys (which would have carried outlet glaciers), mountaintops inundated at the LGM, and nunataks.  The ice surface in the interior was around 1000m, and it descended from c 900m through the studied zone to around 700m.  The ice surface gradient makes sense, indicating a rather gentle gradient in the ice sheet interior ( c 5m per km), then a steeper gradient as the ice flowed through the western mountains (around 15m per km), and then a gentler gradient again towards the coastal lowlands (around 7.5m per km).

But the interesting thing in this research, for those of us who are interested in glaciations other than the late Devensian, is the occurrence of erratics in blockfields ABOVE the trimline, on many mountain summits. Erratics occur up to 900m on the SE ridge of An Teallach, c 140m higher than the reconstructed Late Devensian ice surface. Other erratics occur near the summit of Slioch, c 130m higher than the reconstructed ice surface.  However, the upper limit of erratics lies at 880m - 920m around the Wester Ross mountains -- supporting the idea that the highest summits were never glaciated.  So there are three altitudinal zones -- a lower mountain zone affected by Late Devensian ice, a middle zone affected by an earlier glaciation (or maybe several) but not by the LGM, and an upper zone of mountain summits with no traces of overriding ice.

The implications are clear -- suggesting that the Late Devensian glaciation was not the most extensive or intensive of the Quaternary glaciations, at least in NW Scotland.  But we should not be surprised by the idea that this Greatest British Glaciation (Anglian?) pushed southwards beyond its LGM limit, just as it pushed north-westwards in the Wester Ross area onto the islands of the Outer Hebrides. (The Isle of Lewis was by all accounts unaffected by the ice from the main ice sheet during the LGM.)

There are implications for the Bristol Channel area and for the Isles of Scilly.... more of which anon......

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Ballantyne, C. K., McCarroll, D., Nesje, A. and Dahl, S.-O. 1997. Periglacial trimlines, former nunataks and altitude of the last ice sheet in Wester Ross,northwest Scotland.
J. Quaternary Sci., Vol. 12, pp. 225–238.

ABSTRACT: 

High-level weathering limits separating ice-scoured topography from frost-weathered detritus were identified on 28 mountains in Wester Ross at altitudes of 700–960 m, and a further 22 peaks support evidence of ice scouring to summit level. Weathering limits aredefined most clearly on sandstone and gneiss, which have resisted frost shattering during the Late Devensian Lateglacial, but can also be distinguished on schists and quartzite. Schmidt hammer measurements and analyses of clay mineral assemblages indicate significantly moreadvanced rock and soil weathering above the weathering limits. The persistence of gibbsite above weathering limits indicates that they represent the upper limit of Late Devensian glacial erosion. The regular decline of weathering-limit altitudes along former flowlines eliminates the possibility that the weathering limits represent former thermal boundaries between protective cold-based and erosive warm-based ice. The weathering limits are therefore interpreted as periglacial trimlines that define the maximum surface altitude of the last ice sheet. Calculated basal shear stresses of 50–95 kPa are consistent with this interpretation. Reconstruction of ice-sheet configuration indicates that the former ice-shed lay above 900 m along the present watershed, and that the ice surface descended northwestwards, with broad depressions along major troughs and localised domes around independent centres of ice dispersal. Extrapolation of the ice surface gradient and altitude suggests that the ice sheet did not overrun the Outer Hebrides, but was confluent with the independent Outer Hebrides ice-cap in the North Minch basin. Erratics located up to 140 m above the reconstructed ice surface are inferred to have been emplaced by a pre-Late Devensian ice sheet (or ice sheets) of unknown age.

https://www.academia.edu/30873951/Periglacial_trimlines_former_nunataks_and_the_altitude_of_the_last_ice_sheet_in_Wester_Ross_northwest_Scotland?email_work_card=view-paper